FIG. 1 shows a cross-section of a conventional InGaAsP semiconductor laser device shown in, for example, Japanese Published Patent Application No. SHO 63-202985. A P-type InP cladding layer 32 is disposed on a P-type InP substrate 31. On the P-type InP cladding layer 32, an InGaAsP crystalline active layer 33 is disposed, and on the active layer 33, an N-type InP cladding layer 34 is disposed. An N-type InP buried current blocking layer 35 and a P-type InP buried current blocking layer 36 surround the P-type InP cladding layer 32, the InGaAsP active layer 33 and the InP cladding layer 34 which are disposed in a stack. An N-type InP contact layer 37 is disposed on the N-type InP cladding layer 34. Regions 38 shown by dotted-line circles are P-type regions formed by reversing the conductivity type of the N-type InP blocking layer 35 so that the N-type InP blocking layer 35 does not contact the N-type InP cladding layer 34.
In comparison with AlGaAs semiconductor laser devices, an InP semiconductor laser device including an active layer comprising InGaAsP crystals as described above, has a laser oscillation threshold current which is highly sensitive to temperature at a light-emitting point. This high sensitivity to temperature is considered to be attributable to some causes including non-emissive recombination due to the Auger effect, and overflow of injected carriers. For overcoming this problem, the buried structure as shown in FIG. 1 is frequently employed in InP semiconductor laser devices. With such a structure, the NPNP junction structure provides a dual current blocking effect as indicated by a line A-B in FIG. 1, so that current leakage can be minimized. Thus, current can be injected into the active layer 33 with high efficiency so that laser oscillations with a low threshold current and at high temperature can be provided.
In the conventional semiconductor laser device of the structure shown in FIG. 1, however, the NPNP junction current blocking capability could be lost. That is, depending on carrier concentrations and thicknesses of respective layers, the NPNP junction structure enters into thyristor conduction when the laser device is in a certain operating condition. This causes an increase in leakage current, which, in turn, causes an increase in threshold current and degradation of the temperature characteristics of the device.
Particularly critical structural parameters are the thicknesses and carrier concentrations of the N-type InP blocking layer 35 and the P-type InP blocking layer 36 which provide a junction corresponding to a collector junction of a dual-terminal NPNP thyristor structure. More specifically, if the thickness of the respective blocking layers is on the same order as or less than the diffusion length of minority carriers injected due to thermal excitation or the like, the number of minority carriers which move over barriers increases, causing an increase of leakage current and degradation of the temperature characteristic of the device. This is particularly significant when the device is operated at high temperature.
Generally, in order to minimize increases in the leakage current, the N-type InP blocking layer 35 and the P-type InP blocking layer 36 are both doped to a high concentration of 5.times.10.sup.18 cm.sup.-3 or higher and have a thickness of 1 .mu.m or more. However, in the vicinity of the reversed-conductivity P-type regions 38, the thickness of the N-type InP blocking layer 35 is significantly smaller than that of the remaining portion, and the thickness is smaller than the minority carrier diffusion length. Thus, these regions could cause switching of the device into thyristor conduction. In addition, the width of the reversed-conductivity P-type regions 38 can hardly be uniform, and the magnitude of the leakage current is highly dependent on the location, relative to the active layer 33, of the tip end portions of the current blocking layer 35 in the vicinity of the reversed-conductivity P-type regions 38. Like this laser structure, conventional laser devices such as the one shown in FIG. 1 include unstable structural factors as stated above, which are the major causes for low productivity, a low yield, and low reliability.
An object of the present invention is to provide an improved semiconductor laser device free of the above-described problems, and another object is to provide a method of manufacturing such an improved semiconductor laser device. The semiconductor laser device of the present invention includes an NPNP current blocking structure with stable current blocking capability so that the structure rarely enters into a thyristor conduction state.